分期状态变更通知

Webhook endpoint called by QI Tech when an installment status changes. This notification is used to keep the BNPL system in sync with the financial institution.

Common status transitions:

• PENDING -> PAID: Normal on-time payment
• PENDING -> OVERDUE_GRACE: Payment missed past due date (no fines yet, BNPL limit zeroed)
• OVERDUE_GRACE -> OVERDUE_PENALTY: Grace period expired, fines and interest now apply
• PENDING -> PAID_EARLY: Payment before due date
• OVERDUE_PENALTY -> PAID_OVERDUE: Late payment with fine/interest
• OVERDUE_PENALTY -> PAID_PARTIAL_OVERDUE: Partial late payment
• OVERDUE_GRACE -> PAID: Payment received during grace period (no fines)
• PENDING -> WAITING_PAYMENT: Payment initiated but not confirmed